1. Field of the Invention
This invention relates to a progressive draw control system for a process line.
2. Description of the Prior Art
In a process line, such as paper, for example, a strip of material is passed successively between sections or stands arranged in tandem. Each stand comprises at least a pair of opposed rollers through which the material is passed. Usually the sections or stands are driven by their own speed regulated motor. The basic production speed is set by a master reference voltage which is sent to each section or stand speed regulator where a vernier control is arranged to modify the master reference voltage to provide the desired section or stand speed. The speed difference between stands in the paper industry is called draw and the vernier control (usually a potentiometer) is called draw control.
In such a process line it is important to maintain constant tension in the strip between sections or stands at all time. Perturbations occur which effect interstand tension whenever either the basic production speed, i.e., the master voltage reference, is changed, or a discrete section speed is changed in order to increase the tension between any two sections or stands.
The prior art has been able to cope with disturbances resulting from changes in line voltage, i.e., changes in the basic production speed. The perturbations resulting from changes in section speed in order to increase tension, however have either not been mitigated at all or, where some sort of solution has been achieved, it applied only to a special situation, i.e. the solution did not satisfy the general case. The prior art solutions are discussed in depth hereinafter under the caption "General Considerations."